Differential steer system are well known in the art for use on skid steer machines and track type machines. The differential steer principle is based on providing an input to a differential of a machine that effectively increases the rotation speed of one output shaft and decreases the rotational speed of the other output shaft. The difference in rotational speeds between the respective output shafts causes the machine to turn in the direction of the slowest output speed. The degree of turn is determined by the degree of difference in rotational speed between the respective output shafts. The input to the differential can be increased to a level that results in one of the outputs reaching a zero speed which causes the machine to pivot about the wheel or track that is not turning. Likewise, the input to the differential can be increased to a level that causes one of the outputs to turn in the opposite direction and to a rate equal to the rotational rate of the other output. When one output is rotating in one direction and the other output is rotating in the other direction at the same rate, the machine is negotiating a spot turn.
In the vast majority of the known types of differential steer systems, the input to the differential is provided through a fluid motor. Many of these systems use a dedicated variable displacement pump that is directly connected to the fluid motor in a closed loop manner, commonly referred to as a hydrostatic drive loop. In these systems, the power needed to turn the variable displacement pump, even at zero displacement, takes horsepower from the engine. In machines that do not require frequent steer inputs, the horsepower required to turn the pump is wasted. Consequently, the overall efficiency of the machine is reduced.
In other differential steer systems, the fluid motor is connected to a dedicated pump through a directional control valve. In these systems, the pump is operating at a predetermined pressure level and producing a constant flow rate. When the steer input motor is not being used, the pressurized flow from the pump is being returned to the reservoir across a relief valve or some other type of control valve. Again, the horsepower needed to operate the pump at the predetermined pressure level is wasted. Additionally, in these systems, the flow being returned to the reservoir creates heat which requires extra cooling capacity to keep the oil temperature at an acceptable level. The wasted horsepower and the need for increased cooling capacity decreases the overall operating efficiency of the machine.
In other systems, the pressurized fluid needed to operate the steer input motor is taken from the main system pump, such as the implement pump in order to eliminate the dedicated pump. However, in these types of systems, it is normally necessary to provide a larger pump in order to meet the combined flow demands of both the implement system and the steer input motor. Larger pumps require more horsepower to drive them. As noted above, larger pumps normally require larger cooling systems to maintain the temperature of the oil at an acceptable level. Consequently, the overall operating efficiency of the machine is decreased.
The present invention is directed to overcoming one or more of the problems as set forth above.